Collision warning and intelligent cruise control systems generally employ a forward looking radar (FLR) sensor mounted at the forward end of the host vehicle for acquiring data corresponding to the range, range rate, and the azimuth angle of a target vehicle or other object. A microprocessor receives and analyzes the sensor data along with other data such as vehicle velocity, and predicts the likelihood of an impending collision. In a collision warning system, the primary function of the system is to warn the operator of a potentially unsafe operating condition, or possibly to initiate a corrective action, whereas in an intelligent cruise control system, the primary function is to adjust the vehicle speed to maintain a desired headway or following distance.
It has been recognized that proliferation of such radar-based systems increases the likelihood of system-to-system interference. A typical scenario is illustrated in FIG. 1A, which depicts three vehicles A, B and C travelling on a two-lane roadway 10. Vehicles A and B are travelling in the same direction, and vehicle A is equipped with an ICC system having a scanned, narrow beam, Frequency Modulated Continuous Wave (FMCW) FLR sensor, indicated by the beam 12 and the viewing angle 14. In the illustration, the ICC system is causing vehicle A to pace vehicle B at a controlled distance or range R. Vehicle C, travelling in the opposite direction, is equipped with a fixed, wide beam (or multiple fixed beam), Continuous Wave (CW) FLR sensor, as indicated by the beam 16. The FLR systems in vehicles A and C each receive and process time-delayed reflections of their respective transmitted waveform to determine the range and velocity of potential targets, but in certain cases, the two transmitted waveforms are close enough in frequency to be received as a reflected signal. Exemplary waveforms of the vehicles A and C are graphically depicted in FIG. 1B, where the FMCW and CW waveforms are designated by the traces 20 and 22, respectively. When the waveforms periodically coincide in frequency (i.e., at times t.sub.1, t.sub.2, t.sub.3, t.sub.4), the FLR receiver in vehicle A will output a short duration transient pulse in the time domain, as designated by the reference numeral 18 in FIG. 1C, which graphically depicts the I and Q outputs of a quadrature FLR receiver over a number of samples.
Various techniques have been proposed for mitigating the effects of the system-to-system interference described above. For example, it has been proposed to selectively polarize the transmitted waveforms, or to provide multiple beams with adaptive nulling between beams, or to use spread spectrum waveforms. In general, however, these techniques require undesirable system level tradeoffs and/or significantly increase system cost and complexity.